The present invention relates to electrical switching apparatus control modules and specifically to electrical control circuits for use with electrical transfer switches or load banks having solenoids to selectively energize an electrical distribution system from a plurality of electrical sources.
In supplying electrical power to industrial and commercial facilities, it is often desirable to provide alternate sources of electrical power to insure continuity of service. Sometimes these sources may comprise separate feeder circuits from one or more municipal electric utility companies. In other situations, one or more alternate or non-utility based power sources, such as diesel engine powered generators may be provided as alternate power sources. To change from one power source to another a switch must be provided to switch the distribution system between the alternate sources, and it is often desirable to provide this switching capability as an automatic function. Thus, if the primary power sources should fail, a transfer switch system or a transfer switch with an associated control system will automatically switch the distribution system from the primary power source to the alternate power source.
A load bank is commonly located within the building into which each electrical power source is connected. Accordingly, the load bank has an input connected to the municipal utility power source, an input connected to the alternate power source, and an output connected to the building""s electrical system or load.
The load bank further includes a switching mechanism, commonly known as a transfer switch, in order to switch the input between the power sources. In the past, the switching mechanism has been known to comprise an electromechanical device where an electrical motor is utilized to actuate or switch the load bank from one input source to the other. The electrical motor and associated linkage perform the work required to disconnect the primary power source and connect the secondary power source. The drawback of this arrangement is that the cycle time is relatively slow. In some applications, near instantaneous switch movement is preferred, especially in an electrical system where it is critical that the building distribution system is not interrupted, as in a hospital for example. It is also known in the art to replace the electromechanical system with a solenoid-mechanical system where an electric solenoid is utilized to switch between the power inputs. Each half of the transfer switch requires a dual action (two-pole) solenoid, two single action (single pole) solenoids, or a single action solenoid coupled with an inertial (flywheel) mechanism. To toggle back and forth from one power source to the other, the solenoid configuration must be capable of actuating or throwing the load bank switch in two directions.
Transfer switches are produced in a variety of voltage and current ratings and with varying numbers of poles or parallel electrical paths. Amongst these variations there are two major categorizations of switch systems, open transition and closed transition. An open transition switch disconnects the load from its first source of power before it makes connection to the second source of power. A closed transition switch connects the load to its second source of power before disconnecting from its first source of power. The closed transition switch requires more precise monitoring, control systems, and speed of operation to perform its task properly than an open transition switch. It provides the benefit of being able to transfer the load between electrical sources without an interruption in power. One common situation in which this is very beneficial is when testing backup power systems. This allows the switching mechanism to be cycled as a test without causing a power interruption at the load.
A solenoid is a common electrical device used to convert electrical energy into mechanical energy. Solenoids are well known in the art and are often utilized as a means of moving a component a predetermined distance at a predetermined time. In its most basic form, a solenoid is an electromechanical device that converts electrical energy into a mechanical motion, e.g. linear or rotary motion. Electrical current passes through a coil of insulated copper wire producing a magnetic field, which moves a ferro-magnetic plunger located within the core of the coil. Steel parts surround the coil to provide a flux path for maximum pull, push or rotational force. A solenoid can be used to open a valve, activate a switch, apply a brake or a number of other activities where mechanical movement is required and an electrical energy source is available.
A typical solenoid comprises a steel frame or shell that surrounds the coil of wire and directs the flux path. The coil assembly, when energized with an electrical voltage, creates the magnetic lines of force. A plunger, located within the coil assembly, reacts to the magnetic pull and moves to the center of the coil against a stop or pole piece. The pole piece provides a stop for plunger movement.
The problem that the present invention addresses is that different buildings are supplied with different input voltages and it has been necessary to select a solenoid and an associated solenoid control circuit such that the load bank switch could be operated within the available voltage range. For example one building may receive electricity at the 208 volt level where as another receives electricity at the 480 volt level or some voltage in between. In this instance, each respective load bank would require a different solenoid and different solenoid control circuit designed specifically for the voltage level.
In providing an automatic transfer control device for a specific application, it was usually necessary to engineer a custom design for each application, selecting various solenoids and control modules. It was also necessary to stock the specific control module and solenoid for each transfer control device for use as a replacement part. Accordingly, a great number of different control modules and solenoids were required to be stocked as replacement parts. The present invention provides for a single application over a great range of voltages thereby reducing the number of replacement parts needed to be stocked or kept on hand. Furthermore it is the nature of the method of control utilized in the present invention that also stabilizes the performance of the solenoid when source voltages drift significantly astray of their nominal values and/or when the solenoid coil resistance changes due to temperature changes.
The present invention also embodies such enhancements and conveniences as provision for receiving command/control signals directly from the low power output of programmable logic controllers (PLCs) or other microcomputer based control systems. It provides optical isolation of the control signal firm its own active circuitry. Additionally, the present invention incorporates a timing control which shuts off power to the solenoid after a preprogrammed period of time such that even if external control means should falter, the solenoid and controller will not be damaged from needless sustained high power dissipation.
It is an object of the present invention to provide a solenoid control system that operates over a range of 146 to 576 volts AC applied to one or both (normal and emergency) AC source inputs. This implies the further requirement that the controller, under the condition of both AC sources being active and potentially 90 degrees out of phase with one another be capable of operating with the rms value of the applied input voltage exceeding 700 volts. Solenoids provide faster operation than prior electric motors. Properly controlled, solenoid actuation occurs in less than 100 milliseconds. The simple mechanics of a solenoid-based system provides higher reliability. Furthermore, the fail neutral behavior of solenoids is particularly suitable to transfer switches because they require manual override capability. It is an object of the present invention to provide a solenoid and control module system that can operate over a wide range of voltages and over a wide range of temperatures without significantly varying in performance. This stabilizes and allows more optimization of system performance and additionally significantly reduces inventory requirements.
Solenoids can incorporate internal stops to remove shock load from the switch mechanism. It is an object of the present invention to provide a solenoid control system that provides an optically isolated interface directly to the master controller or programmable logic controller. No relays are needed. It is an object of the present invention to provide a solenoid control system that incorporates robust transient protection. The control module comprises an automatic timing control that protects the solenoid from burn-out and eliminates the need for end-of-travel limit switches. The compact design and size of the control module and solenoid saves space. The invention achieves current regulation through a very simple and low cost circuit that capitalizes on the conduct inherent in the solenoid load.
It is an object of the present invention to provide a solenoid control system utilizing a method of current sensing and the selection of components that eliminates the need to perform any post assembly adjustments to achieve usable regulation accuracy. The timing and regulation control portion of the circuit operates with a quiescent current requirement of only a few milliamps yet still is able to control very high power load banks. It does so through very efficient circuit design and the novel use of CMOS timer integrated circuits (ICs) as efficient insulated gate bipolar transistors (IGBTs) gate drivers. Finally, combined use of a resistor and capacitor (RC) snubber circuit and transient voltage suppressors protect the switching semi-conductors from high unclamped inductive energy levels associated with fast switching of currents through AC supply sources and power grids.
These and other objects are achieved by the present invention wherein an electrical control module supplies regulated current to each section of a dual action (two-pole) solenoid or to single action (one-pole) solenoids.
In one embodiment, the invention may be described as a system for transferring electrical power from a first power source to electrical power from a second power source having a transfer switch including first and second inputs and an output, the first electrical power source being connected to the first input and said second electrical power source being connected to the second input, a rectifier connected to at least one of the power sources, at least one solenoid, the solenoid mechanically coupled to the transfer switch, at least one solenoid control circuit, the solenoid control circuit being electrically connected to the rectifier and solenoid, and a controller, the controller being connected to the solenoid control circuit whereby the controller sends a signal to the solenoid control circuit thereby causing the solenoid to move the transfer switch from one of said power sources to the other.
In another embodiment, the invention comprises a solenoid control circuit connected to first and second voltage sources, connected to an external signal mechanism and connected to a solenoid having at least one coil. The circuit includes a control means for receiving and validating a control signal from the external signal mechanism. A solenoid current regulator, including a current monitoring circuit, is coupled to the control means, is capable of receiving the control signal and is coupled to a micro-controller means. A power switching means is connected to and receives power from one of the voltage sources. The micro-controller is connected to and provides a current signal to the power switching means which is in turn connected to the solenoid coil. The current signal allows the current flowing through the solenoid to decay for a predetermined period of time after a predetermined current level is established in the solenoid coil.
In a third embodiment, a circuit for controlling the current applied to a solenoid having an external signal source and a voltage source including a signal receiving means, a power switching means and a current regulating solenoid driver circuit. The signal receiving means receives a signal from the external signal source. The power switching means is connected to the voltage source and the solenoid. The current regulating solenoid driver circuit is capable of monitoring the signal from the signal receiving means and sending an output to said power switching means thereby allowing the current flowing through the solenoid to decay for a predetermined period of time after a predetermined current level is established in the solenoid coil.